Split AST Declaration class, in preparation for new module declaration forms.

Split AST Declaration class, in preparation for new module declaration forms.

Turns Declaration into an abstract class, and introduces VariableDeclaration as a concrete subclass.

R=kmillikin@chromium.org
BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/9348057

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10662 ce2b1a6d-e550-0410-aec6-3dcde31c8c00